The present invention relates to a photoelectric conversion device, which generates photoelectromotive force by irradiation of light, and a process for producing a photoelectric conversion device. The photoelectric conversion device herein involves various photoelectric conversion devices, such as a solar cell, an image sensor, a photo sensor, as specific application fields.
As a photoelectric conversion device using an amorphous semiconductor film as a photo active layer, a solar cell, an image input sensor (image sensor) and an photo sensor have been practically used. Particularly, a solar cell using an amorphous silicon film has been known as a power source of a desk calculator, and has been widely used.
An amorphous silicon film has characteristic features, comparison to a crystalline silicon material, in that a film having a large area can be produced at a low temperature of 400xc2x0 C. or lower, and the thickness that is sufficient to absorb light as a photoelectric conversion layer is as thin as about 1 xcexcm. Therefore, saving of the silicon resource and the energy required for its production can be expected, and it has attracting attention as a material of low cost in comparison to the conventional materials.
A diode structure having a pin junction has been generally used in the field of a solar cell, an image sensor and a photo sensor, for improving photoelectric conversion efficiency and photo response property. While p type, i type and n type layers all can be formed with amorphous silicon films, it has been known that in order to obtain good photoelectric conversion characteristics, the p type and n type semiconductor layers are formed with microcrystalline silicon films. This is because as the photoelectric conversion is mainly performed in the i type layer by light absorption, the p type and n type layers preferably have high light transmission property and is preferably formed with a material having high conductivity to attain good contact with an electrode. A microcrystalline silicon film has low light absorption loss and high conductivity, and is suitable as the material for the p type and n type layers.
An amorphous silicon film is produced by a chemical accumulation process using glow discharge plasma under reduced pressure (plasma CVD process) A plasma CVD apparatus, which is composed of a reaction chamber, an evacuation means for maintaining the reaction chamber under reduced pressure, a gas introducing means for introducing a raw material gas, a means for generating glow discharge plasma in the reaction chamber, and a means for holding and heating a substrate, is used. While silane (SiH4) is generally used as the raw material gas, disilane (Si2H6) gas can also be used. The raw material gases may be after diluting with hydrogen (H2) gas.
A microcrystalline silicon film is produced by using a mixed gas of SiH4 gas and H2 gas, as a raw material gas, under the conditions in that a diluting ratio of the H2 gas is higher than the SiH4 gas. It has been known that a microcrystalline silicon film, to which no impurity element determining the conductive type, p type or n type, is added, exhibits n type conductivity. In general, an impurity gas containing an element determining the conductive type, p type or n type, is added on formation of the film to further improve the conductivity and to control the conductive type of p type or n type.
In the field of semiconductors, elements represented by boron (B), aluminum (Al), gallium (Ga) and indium (In), which belong to the IIIb group in the periodic table, have been known as an element determining p type conductivity, and phosphorous (P), arsenic (As) and antimony (Sb), which to the Vb group in the periodic table, have been known as an element determining n type conductivity. In the general plasma CVD process, an impurity gas represented by B2H6 and PH3 is mixed with the raw material gas on the film formation. The addition amount of the impurity gas added is generally about from 0.1 to 5% relative to SiH4, and about 10% at most.
Because the microcrystalline silicon film and the amorphous silicon film have a low process temperature, an organic resin material can be used as a material for a substrate of a photoelectric conversion device, in addition to a glass material.
In the basic process for production of a solar cell and an image sensor formed on a substrate, a first electrode is formed on the substrate, a photoelectric conversion layer composed of a pin junction is formed on the first electrode in intimate contact therewith, and a second electrode is accumulated thereon. In the formation of the pin junction, the process is continued without breaking vacuum to improve the characteristics of the junction boundary.
At this time, it has been known that when the impurity gas is added to the raw material gas to form a p type or n type semiconductor film, a slight amount of the impurity gas and its reaction product adhere on the reaction chamber and a discharge electrode, which is a part of the means for generating glow discharge plasma. When an i type amorphous silicon film, which is substantially intrinsic, is continuously formed without addition of any impurity gas, there is a problem in that the residual impurity is released and incorporated into the i type amorphous silicon film. The substantially intrinsic i type silicon film is produced to have a defect density in the film of 1xc3x971016 per cubic centimeter, and thus if the impurity element is incorporated in a concentration of from several tens to several hundreds ppm, it forms an impurity level to change the characteristics of the film.
In order cope with such a problem, a plasma CVD apparatus of separated multichamber type has been developed, in which plural reaction chambers are provided, which are separated from each other by a gate valve provided between the reaction chambers. Therefore, in order to produce the pin junction according to the conventional process, at least three reaction chambers for forming the p type, i type and n type semiconductor layers.
As a result, a gas introducing means for introducing SiH4, H2, B2H6 and PH3, an evacuation means, and a glow discharge plasma generation means must be provided in each of the reaction chambers, and thus the constitution of the apparatus becomes complicated and exaggerated. The maintenance of the apparatus requires much labor, accordingly.
From the standpoint of the production process, after a semiconductor film of one conductive type is formed, the substrate must be moved from one reaction chamber to another reaction chamber, and the introduction and evacuation of the reaction gas must be conducted. These procedures must be repeated in order. Therefore, the reduction in time required to form the photoelectric conversion layer is naturally limited. Even when a technique of high-speed film formation is employed to reduce the time required for forming a film, the time required for transfer off the substrate and introduction and evacuation of the gas becomes a bar in reduction of the production time.
In the conventional technical field of a solar cell, it has been known that the concentration of a p type impurity in a p/i boundary of a photoelectric conversion layer is continuously changed to form a continuous boundary for improvement in junction characteristics. In order to realize such a technique in the conventional process, a gas containing a slight amount of a p type impurity element must be precisely controlled by using a computer.
In order to solve the above-described problems, the invention provides a process for producing a photoelectric conversion layer, in which a step of forming an amorphous semiconductor layer and a microcrystalline semiconductor layer, and a step of adding an impurity to the microcrystalline semiconductor layer to control the conductive type are separated, and also provides a photoelectric conversion device produced by the process.
As a result of experimentation by the inventors, it has been found that a p type or n type microcrystalline silicon film can be obtained, in the conventional film forming process by using the plasma CVD process, by forming a microcrystalline silicon film without adding a p type or n type conductive type determining impurity element, and then injecting the p type or n type conductive type determining impurity element to the microcrystalline silicon film by using an ion doping method, followed by heat activation, as a method for controlling the conductive type of the microcrystalline silicon film. While it has been known that a microcrystalline silicon film containing no p type or n type conductive type determining impurity element exhibits n type conductivity, the conductivity is further increased by conducting the heat activation step.
Therefore, in the invention, the addition of the p type or n type conductive type determining impurity element is not required in the step of forming a p type or n type microcrystalline silicon film. Thus, in the steps of forming a p type or n type microcrystalline silicon film and a substantially intrinsic amorphous silicon film (i type amorphous silicon film), the contamination by the impurity described above cannot be ignored, and the films can be continuously formed in one reaction chamber.
According to the invention, microcrystalline silicon films having p type conductivity and n type conductivity can be obtained by the step of injecting a p type conductive type determining impurity element to a microcrystalline silicon film, and a step of heat treating such a microcrystalline silicon film added with the p type conductive type determining impurity element and a microcrystalline containing no p type or n type conductive type determining impurity element. That is, there is no necessity of using an n type conductive type determining impurity element.
The invention is characterized in that in the production process of a photoelectric conversion layer represented by the plasma CVD process, the process comprises, in order to improve the production efficiency, a step of forming, in intimate contact with a first electrode, a photoelectric conversion layer comprising first and second microcrystalline semiconductor films produced without adding p type or n type conductive type determining impurity element, and a substantially intrinsic amorphous semiconductor film; a step of forming a second electrode in intimate contact with the photoelectric conversion layer; and a step of injecting a p type conductive type determining impurity element to a surface of the second electrode, followed by heating.
The invention is characterized in that a photoelectric conversion device is produced by a process comprising a step of forming a photoelectric conversion layer comprising a first microcrystalline semiconductor film, a substantially intrinsic amorphous semiconductor film, and a second microcrystalline semiconductor film, and a step of injecting a p type conductive type determining impurity element from a surface of a second electrode provided on the second microcrystalline semiconductor film to the second microcrystalline semiconductor film and the vicinity of a boundary of the substantially intrinsic amorphous semiconductor film and the second microcrystalline semiconductor film, followed by heating.
The invention is characterized in that, in order to prevent contamination of impurity gas in the plasma CVD process, a photoelectric conversion layer is produced by forming first and second microcrystalline semiconductor films without adding a p type or n type conductive type determining impurity element, and heating the first microcrystalline semiconductor film and the second microcrystalline semiconductor film, to which a p type conductive type determining impurity element is injected, to obtain microcrystalline semiconductor films having p type conductivity and n type conductivity.
The invention is characterized in that, in order to improve the productivity in the production process of a photoelectric conversion layer represented by the plasma CVD process, the process comprises a step of forming a photoelectric conversion layer comprising first and second microcrystalline semiconductor films produced without adding a p type or n type conductive type determining impurity element, and a substantially intrinsic amorphous semiconductor film; and a step of injecting a p type conductive type determining impurity element into the first or second microcrystalline semiconductor film, followed by heating so as to be thermally activated.
The invention is characterized in that, in order to improve the productivity in the production process of a photoelectric conversion layer, a step of forming a first microcrystalline semiconductor film without adding a p type or n type conductive type determining impurity element, a step of forming a substantially intrinsic amorphous semiconductor film, and a step of forming a second microcrystalline semiconductor film without adding a p type or n type conductive type determining impurity element are conducted from the side of a first electrode; and then a p type conductive type determining impurity element is injected to the second microcrystalline semiconductor film, followed by heating. At this time, the p type impurity element may be injected to the second microcrystalline semiconductor film, and the vicinity of the boundary between the second microcrystalline semiconductor film and the substantially intrinsic amorphous semiconductor film.
The invention is characterized in that, in order to improve the productivity in the production process of a photoelectric conversion layer, a step of forming a first microcrystalline semiconductor film without adding a p type or n type conductive type determining impurity element, a step of forming a substantially intrinsic amorphous semiconductor film, and a step of forming a second microcrystalline semiconductor film without adding a p type or n type conductive type determining impurity element are conducted from the side of a first electrode; and after forming a second electrode, a p type conductive type determining impurity element is injected to the second microcrystalline semiconductor film, followed by heating. At this time, the p type impurity element may be injected to the second microcrystalline semiconductor film, and the vicinity of the boundary between the second microcrystalline semiconductor film and the substantially intrinsic amorphous semiconductor film.
In the most preferred embodiment of the invention, a microcrystalline silicon film is used as the microcrystalline semiconductor film, and an amorphous silicon film is used as the amorphous semiconductor film. As the amorphous semiconductor film, an amorphous silicon carbide film, an amorphous silicon germanium film, and an amorphous silicon tin film can also be used.
In one preferred embodiment of the invention, an ion doping method is employed as the method for injecting a p type conductive type determining impurity element from the surface of the second electrode.
In one embodiment of the invention, the process is characterized by comprising a step of forming a first electrode on a substrate, a step of forming a first microcrystalline semiconductor film without adding an type or p type conductive type determining impurity element, a step of forming a substantially intrinsic amorphous semiconductor film, a step of forming a second microcrystalline semiconductor film without adding an n type or p type conductive type determining impurity element, a step of injecting a p type conductive type determining impurity element to the second microcrystalline semiconductor film, a step of subjecting the first and second microcrystalline semiconductor films and the substantially intrinsic amorphous semiconductor film to a heat treatment, and a step of forming a second electrode.
In another embodiment of the invention, the process is characterized by comprising a step of forming a first electrode on a substrate, a step of forming a first microcrystalline semiconductor film without adding an n type or p type conductive type determining impurity element, a step of forming a substantially intrinsic amorphous semiconductor film, a step of forming a second microcrystalline semiconductor film without adding an n type or p type conductive type determining impurity element, a step of injecting a p type conductive type determining impurity element to the second microcrystalline semiconductor film, and the vicinity of the boundary between the second microcrystalline semiconductor film and the substantially intrinsic amorphous semiconductor film, a step of subjecting the first and second microcrystalline semiconductor films and the substantially intrinsic amorphous semiconductor film to a heat treatment, and a step of forming a second electrode.
In another embodiment of the invention, the process is characterized by comprising a step of forming a first electrode on a substrate, a step of forming a second microcrystalline semiconductor film without adding an n type or p type conductive type determining impurity element, a step of injecting a p type conductive type determining impurity element to the second microcrystalline semiconductor film, a step of forming a substantially intrinsic amorphous semiconductor film, a step of forming a first electrode on a substrate, a step of forming a first microcrystalline semiconductor film without adding an n type or p type conductive type determining impurity element, a step of subjecting the first and second microcrystalline semiconductor films and the substantially intrinsic amorphous semiconductor film to a heat treatment, and a step of forming a second electrode.
Another embodiment of the invention is a photoelectric conversion device comprising a first electrode provided on a substrate, a substantially n type first microcrystalline semiconductor film provided in intimate contact with the first electrode, a substantially intrinsic amorphous semiconductor film provided in intimate contact with the first microcrystalline semiconductor film, a second microcrystalline semiconductor film containing a p type conductive type determining impurity element provided in intimate contact with the substantially intrinsic amorphous semiconductor film, and a second electrode provided in intimate contact with the second microcrystalline semiconductor film.
Another embodiment of the invention is a photoelectric conversion device comprising a first electrode provided on a substrate, a substantially n type first microcrystalline semiconductor film provided in intimate contact with the first electrode, a substantially intrinsic amorphous semiconductor film provided in intimate contact with the first microcrystalline semiconductor film, a second microcrystalline semiconductor film provided in intimate contact with the substantially intrinsic amorphous semiconductor film, and a second electrode provided in intimate contact with the second microcrystalline semiconductor film, a p type conductive type determining impurity element is contained in the second microcrystalline semiconductor film, and the vicinity of the boundary between the substantially intrinsic amorphous semiconductor film and the second microcrystalline semiconductor film.
Another embodiment of the invention is a photoelectric conversion device comprising a first electrode provided on a substrate, a second microcrystalline semiconductor film containing a p type conductive type determining impurity element provided in intimate contact with the first electrode, a substantially intrinsic amorphous semiconductor film provided in intimate contact with the second microcrystalline semiconductor film, a first microcrystalline semiconductor film provided in intimate contact with the substantially intrinsic amorphous semiconductor film, and a second electrode provided in intimate contact with the first semiconductor film.
According to the invention, the addition of a p type or n type conductive type determining impurity element is not necessary in comparison to the conventional process for forming a p type or n type microcrystalline semiconductor film. Thus, in the process for forming a p type or n type microcrystalline semiconductor film and a substantially intrinsic i type amorphous semiconductor film, the above-described contamination of impurities must not be considered, and for example, the films may be continuously formed in one chamber for film formation. Specifically, as a p type and n type microcrystalline silicon films and a substantially intrinsic silicon film are produced by using only SiH4 gas or Si2H6 gas, and H2 gas, they can be formed with one glow plasma generation means provided in one reaction chamber. Furthermore, a p type and n type microcrystalline silicon films and a substantially intrinsic silicon film can be continuously formed with maintaining glow discharge plasma.
According to the invention, because the introduction of an impurity gas in the formation process of a p type or n type microcrystalline semiconductor film and a substantially intrinsic amorphous semiconductor film is not necessary, and the influence of the contamination of the impurity in the plasma CVD apparatus can be reduced, the use of only one reaction chamber is substantially sufficient, and the constitution of the plasma CVD apparatus can be simplified. Furthermore, the time required for transportation of a substrate, and the time required for introduction and evacuation of the gas can be reduced. On the other hand, even when a conventional separated multichamber type apparatus is employed, an impurity gas must not be introduced in the film formation process to prevent contamination of impurities in the film formation process, and thus plural substrate can be simultaneously processed in plural reaction chambers.
Therefore, according to the invention, microcrystalline semiconductor films exhibiting a p type conductivity and n type conductivity, respectively, and a substantially intrinsic amorphous semiconductor film can be obtained by a step of forming a first microcrystalline semiconductor film, a step of forming a substantially intrinsic amorphous semiconductor film, a step of forming a second microcrystalline semiconductor film, a step of injecting a p type conductive type determining impurity element to the second microcrystalline semiconductor film, and a step of heating the first and second microcrystalline semiconductor films and the substantially intrinsic amorphous semiconductor film, and thus a pin junction can be produced. A microcrystalline silicon film, to which no p type or n type conductive type determining impurity element is added, exhibits n type conductivity by itself. It has been found that even when a pin junction is formed by using such a microcrystalline silicon film having n type conductivity, a good photoelectric conversion layer can be obtained. Therefore, an n type impurity element, which is used in the conventional technique, must not be used.
In a preferred embodiment of the invention, because the first and second microcrystalline silicon films and the substantially intrinsic amorphous silicon film can be produced with SiH4 gas and H2 gas, there is no necessity of changing the gases on formation of each of the films. Therefore, the time required for transporting the substrate from a reaction chamber to another reaction chamber, and the time required for introduction and evaluation of the gas, which are needed in the conventional process, are not necessary, and thus the process performance can be enhanced.
According to the invention, the p type and n type microcrystalline silicon films and the substantially intrinsic amorphous silicon film can be simultaneously or separately produced in the reaction chambers of a plasma CVD apparatus having plural reaction chambers, and thus the process performance can be enhanced.
Furthermore, by injecting a p type conductive type determining impurity element from the surface of the second microcrystalline semiconductor film by using the ion doping method, the concentration distribution of the p type impurity in the direction of the film thickness can be easily controlled.
The invention can be applied to a photoelectric conversion device using an organic resin substrate selected from polyethyleneterephthalate, polyethylenenaphthalate, polyethersulfone, polyimide and aramid.